As global reserves of light crude oil diminish, the exploration for and production of heavy oil and bitumen becomes of increased importance to maintain a stable global supply of hydrocarbon. When evaluating heavy oil or bitumen formations, it is advantageous to obtain representative samples of the formation to determine appropriate drilling and production methods. However, due to the mobility of heavy oil and bitumen, sampling these formations can be difficult or impossible using many known light crude oil sampling techniques.
Attempting to sample a heavy oil or bitumen, for example, without first increasing the mobility of these fluids can result in excessive drawdown pressures, which can cause failure of a pump or pumpout unit being used to extract the fluid, failure (e.g., cracking, fracturing and/or collapse) of the formation, and/or phase changes and, thus, compositional changes to the fluid being sampled. Further, such excessive drawdown pressures can lead to the production of sand, which may cause failure of sampling tool seals. While increasing the areas of the sampling ports or probes can reduce the drawdown pressures, larger port or probe areas can be difficult to achieve without adversely impacting the size of the sampling tool and the ability to achieve an effective seal around the sampling ports or probes.
One factor contributing to the low mobility of heavy oil and bitumen formation is the high viscosity of these fluids. As illustrated by Equation 1 below, a flow-rate of fluid from a subsurface formation may be changed by increasing a pressure difference, changing the permeability of the formation or by decreasing the viscosity of the formation fluid. The pressure difference applied by the sampling tool to withdraw the fluid is represented by Δp, the fluid viscosity is represented by η and the permeability of the formation is represented by k.Q∝Δp·k/η  Equation 1
Substantially reducing the viscosity of the heavy oil and bitumen in a formation can increase mobility sufficiently to obtain a sample. However, to be helpful in determining a production strategy, the fluid sample has to be representative of the formation fluid and/or any changes to the characteristics of the fluid sample have to be reversible.
Some known methods to increase the mobility of formation fluids involve heating the formation through a variety of means (e.g., thermal stimulation), or injecting a diluent into the formation (e.g., non-thermal stimulation). The diluent or solvent is usually miscible with the formation fluid, and in these cases, the diluent may be referred to as a solvent. However, steam or water may not be readily miscible diluents. Production methods that rely on injecting a suitable solvent into a formation include vapor assisted extraction (VAPEX). Another primary production method is cold heavy oil production with sand (CHOPS) that relies on reducing the pressure and evolving the gas from the formation to produce a foam. Some example methods of heating a formation include cyclic steam circulation, steam floods, and steam assisted gravity drainage (SAGD). While the use of some diluents may be appropriate for certain applications such as, for example, production in which the chemical composition and/or the physical properties of the formation fluid need not be maintained, these diluents may not be appropriate to obtain samples of formation fluid because they irreversibly change the formation fluid.
While the above-mentioned methods may be used to change the mobility of a formation fluid, in some circumstances, the mobility of the formation fluid is not sufficiently increased by either heating the formation fluid or injecting a diluent into the formation fluid.